A volcano named Sapas Mons dominates this computer-generated view of the surface of Venus. Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground to the base of the mountain, which measures 248 miles (400 km) across and 0.9 mile (1.4 km) high.

It’s got to be frustrating for planetary scientists that the world closest to Earth and most similar in size as well—so similar that it’s often described as our planet’s twin—is in many ways the toughest to study. Venus’ perpetual shroud of thick, opaque clouds makes it impossible to see the surface with even the most powerful telescopes, and its sweltering surface temperature (think 900°F) makes it impossible for landers to survive for more than a few minutes.

Nevertheless, orbiting spacecraft armed with radar and other instruments have taught scientists quite a lot about Venus’ atmosphere over the years, and even its surface—and a new study in Nature Geoscience is now reporting another, intriguing piece of information. The European Space Agency’s Venus Express has detected a sharp decline in sulfur dioxide (SO2) concentrations in the planet’s atmosphere following a spike in SO2 just after the probe arrived in 2006. The most plausible explanation, says lead author Emmanuel Marcq, is a volcanic eruption, caught in the act.

It’s not the only possible explanation, he admits. “We know that on Earth there are long-term atmospheric cycles, so it could happen on Venus as well. We can’t dismiss this possibility at the moment.”

Volcanoes, however, are a lot more likely. One reason is that they’re known sources of SO2, at least on Earth. Another is that Venus is peppered with them — hundreds upon hundreds of volcanic mountains, lava flows and other features that make the planet’s history of volcanism unmistakable. Most of them appear to be long dead, but there have been hints in the past that a few might still be active. In the early 1990’s for example, radar on the Magellan spacecraft detected what appeared to be fresh lava, and back in the early 1980’s Pioneer Venus documented SO2 levels nearly 50 times higher than anyone expected, followed by a steady dramatic decline. “It’s very similar,” he says, “to the one we’re observing now.”

If a volcano really did erupt just after Venus Express showed up, the event could have been seen directly by the satellite’s infrared detectors — but only on the planet’s night side, since average daytime temperatures are so high that an extra hotspot wouldn’t stand out very easily. And even so, the orbiter only passes over a given spot on the Venusian surface every 100 days. “It’s unlikely,” says Marcq, “that we would be able catch an eruption.”

It’s also highly unlikely that this eruption, or any conceivable eruption on Venus, could do much to make the planet more hospitable. When a big volcano goes off on Earth — Krakatoa, for example, or even the more modest Pinatubo — its sulfur dioxide output forms light-reflecting particles high in the stratosphere that bounce sunlight back into space and temporarily cool the planet. Indeed, volcanic eruptions are one of the natural forces that climate scientists take into account when trying to discern the fingerprint of manmade climate change.

On Venus, however, the carbon-dioxide-rich atmosphere has created a runaway greenhouse effect and global warming beyond anyone’s worst nightmares. This recent eruption could cool the planet – but only by a few degrees. When you’re talking temperatures that are already hot enough to melt lead, a few degrees of relief isn’t going to make a whole lot of difference for vulnerable rovers – or astronauts.